test: improve coverage for process.umask

This ensures that process.umask() throws with the correct error when
invalid inputs are supplied.

PR-URL: nodejs/node#16188
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-umask.js

@@ -40,3 +40,11 @@ assert.strictEqual(parseInt(mask, 8), process.umask(old));
 assert.strictEqual(old, process.umask());
 // 2. If the test fails, process.umask() will return 0
 assert.strictEqual(old, process.umask());
+
+assert.throws(() => {
+  process.umask({});
+}, /argument must be an integer or octal string/);
+
+assert.throws(() => {
+  process.umask('123x');
+}, /invalid octal string/);